Vehicles use a wide variety of transmissions for vehicle propulsion. A type of transmission that is often used in smaller vehicles, e.g., snowmobiles, go-karts, and all terrain vehicles (ATV), is an endless belt transmission, often referred to a continuously variable transmission (CVT). In a CVT, both outward torque and speed varies substantially continuously, i.e., without gearshifts, over the entire speed range of the engine. A CVT typically includes a driving clutch having a shaft that is coaxial with the output shaft of the vehicles engine. The driving or primary clutch includes a fixed sheave and a movable sheave that together define a pulley around which a drive belt travels. The drive belt also engages a driven or secondary clutch that transfers the engine's clutch power to a secondary shaft. The driven clutch also includes a fixed sheave and a movable sheave that together define a pulley.
The effective radius of both the primary and the secondary pulley may be variable. The ratio of the primary pulley radius to the secondary pulley radius determines the ratio of engine rotational speed to the secondary shaft rate of rotation. When the primary clutch radius is smaller than the secondary clutch radius, the secondary shaft will turn at a rate that is slower that the engine speed, resulting in a relatively low vehicle speed. As the ratio of the primary and the secondary clutch radius approaches 1:1, the secondary shaft speed will be approximately equal to the engine or crankshaft speed. As the primary pulley radius becomes greater than the radius of the secondary clutch, an overdrive condition exists in which the secondary shaft is turning at a greater rate than the engine crankshaft.
The primary clutch is connected to the power source and in theory has the job of maintaining the engine rpm at a value where the most power is being produced by the engine. The primary clutch may also control engagement and disengagement of the engine from the load in order to stop and start vehicle movement. In the case of a snowmobile, the secondary or driven clutch is connected to the load through a jackshaft, gears, chain and track, and functions to change the ratio of the two clutches as the load varies. This function is performed by a torque sensing helix or the like, that is typically considered part of the secondary clutch. An example of a secondary clutch having a torque sensing helix is disclosed in U.S. Pat. No. 5,516,333.
As the load to the secondary clutch fluctuates, the torque sensing helix will balance the power being received from the engine and the load by widening or narrowing the distance between the clutch sheaves. Altering the distance between the clutch sheaves changes an effective radius of the clutch around which the drive belt travels. The torque sensing helix is intended to automatically make widening and narrowing adjustments (upshifts and downshifts) almost instantaneously.
The torque sensing helix is essentially a cam slot formed in a clutch housing. The cam slot includes cam surfaces that engage associated cam followers that transfer the adjustments made by the torque sensing helix into variations of width between the clutch sheaves. The fixed sheave of the pulley is typically secured to the secondary shaft that transfers a load to and from the vehicle's track or wheels. The clutch housing including the torque sensing helix is secured to the movable sheave and retains a compression/torque spring against the fixed sheave. The compression/torque spring acts between an end of the housing and the fixed sheave, and is typically adjustable within the housing. The fixed sheave typically has cam followers secured to it that engage the cam surfaces of the torque sensing helix housing. As the torque sensing helix senses a change in load from the secondary shaft, the moveable sheave of the driven clutch will move to either compress or relax the compression spring causing the cam followers to move up or down the cam surfaces of the helix housing to increase or decrease the radius of the driven clutch.
A common problem associated with a driven clutch that utilizes a torque sensing helix with cam followers is backlash. Backlash occurs as the compression/torque spring moves from a relaxed to a compressed position and back to a relaxed position. The compression spring is commonly used to apply both torsion forces and compression forces to the cam followers that contact the cam surfaces. Torsional tension of the spring is applied to keep the cam followers in contact with the cam surfaces during changes in loading operations. In practice, however, the cam followers may actually leave the cam surface, usually when the load has momentarily significantly decreased. If the cam followers do leave the cam surface and the load is suddenly reintroduced, the cam followers recontact the cam surfaces with high, sometimes destructive forces. At the moment of recontact, the driven clutch may be set at an excessively high ratio so that the engine crankshaft speed immediately drops below the desired rpm range, causing a drop in power output from the system.
As mentioned, the compression/torsion spring must include a torsional component in order for the cam followers to maintain contact with the cam follower helical surface. An alternative to a single spring having both compression and torsion properties is to utilize separate springs in the housing, with one spring providing torsion forces and the other spring providing compression forces. However, even with the torsional component included in the housing, whether from one spring or two separate springs, backlash may still occur. Requiring a torsional component in the housing creates certain limitations to the design of the housing, the ability to adjust the spring(s), and the ability for the driven clutch to accommodate different belts having different widths, as well as a variety of other design considerations. A torque responsive clutch addressing disadvantages of known clutches and their components would be an important advance in the art.